We have been unable to confirm acidification of L1210 leukemia cells at VP-16 concentration of 150nM or less. Our continuation of these experiments fails to show acidification of these cells by VP-16 alone at a concentration of up to 420 nM, which is far in excess of their VP-16 1C50 value of 110nM. The addition of Cyclosporin A to VP-16 also fails to promote cellular acidification which indicates that Cyclosporin enhancement of VP-16 cytotoxicity relates to an alternate mechanism.